Because of their unique, excellent properties not generally offered by metals (e.g. hardness, high melting point and chemical stability), ceramic materials have been known to be highly useful materials. Unfortunately, these materials are not only hard but brittle and dielectric or electrically nonconductive so that they are not readily machinable by an existing machining technique, e.g. abrasive grinding or cutting, electrical discharge machining or electrochemical machining. Accordingly, there have been severe limitations in their uses as to expanding their industrial applicabilities.
It is known that ceramics, which are highly stable against chemical attacks under normal conditions, become chemically reactive when placed at an elevated temperature. It has been believed impossible, however, to effectively exploit this fact in machining a ceramic material since no practical means or material was deemed to be available which could, without serious attacks, withstand a chemical heated to an elevated temperature in storing, supplying and recovering it.